The present invention relates to a hydraulic control system for an automatic transmission which utilizes a torque converter, and more particularly to a line pressure control of the hydraulic control system.
Conventionally, in order that the line pressure varies with output torque of an engine and with an increase in torque resulting from a multiplication in torque by a torque converter at low vehicle speeds, the line pressure is regulated to vary with throttle pressure and the magnitude of the line pressure is maintained high until a predetermined vehicle speed is attained and is lowered at vehicle speeds above the predetermined vehicle speed. This control strategy of the line pressure is used to meet torque requirements of friction units such as clutches and brakes.
However, although the line pressure provided by the conventional line pressure control meets the torque requirements of the friction units, the variation of the pattern, of the line pressure does not agree satisfactorily with the pattern of torque on the transmission input shaft which is delivered thereto via a torque converter. This is because the line pressure varies in a disrete manner in the neighborhood of a predetermined vehicle speed, creating an operating range wherein the line pressure is unnecessarily high as compared to the torque requirements of friction units. An oil pump, therefore, has to work harder than necessary within the operating range, resulting in a drop in the efficiency of the entire automatic transmission.